1. Field of the Invention
The invention relates to capacitors and more particularly to electrical wound capacitors.
2. Description of the Prior Art
Electrical wound capacitors are employed for damping pulsating d.c. voltages, whereby support capacitors can briefly emit high currents given periodic peak power consumption and thus support a d.c. voltage network.
The capacitors are particularly employed in power electronics and have nominal voltages .gtoreq.-3 kV. the internal series connection is employed so that such capacitors can be utilized with good operating reliability given overvoltage. A paper tape that has two metal layers separated from one another by a free strip is thereby employed as contacted electrode, whereby the connection to the external electrode terminals ensues via front contact (schoopage) layers. A further paper tape is provided with a metal layer that is not connected to the schoopage layers. Under known conditions, non-metallized paper tapes can be co-wound, these being referred to as filler paper.
A weak point of the internal series connection is the cut or intersecting edges of the contacted electrode with free strips at the beginning and end of the winding. During operationally caused overvoltages, the capacitor windings fail here due to arcing from coating edge to coating edge at the free strip. This arcing is elicited in that the field lines at the beginning and end of the winding lead through a gap having the thickness of a dielectric ply which may be filled with impregnation agent under normal conditions. Since the insulation here is thinner than that of the capacitor paper, the arcing from coating edge to coating edge is facilitated.
It is in fact possible in an attempt to avoid this problem, to burn off the metal layers at the beginning and end of the winding by means of applying a high-frequency voltage, whereby a coating-free area arises. This procedure, however, is time intensive and requires high capital costs at the winding machine. Further, there is the risk that the capacitor paper may be damaged due to discharges during the burn-off.
Further, there would also be the possibility of complicating the arcing by widening the free strip. The capacitance of the capacitor winding, however, would be thereby diminished.